Of the numerous different types of electronic heart pacemakers and their corresponding various modes of operation which have been proposed in the art, the present invention is primarily directed to atrial-ventricular sequential pacemakers operating on a demand basis, either in an implantable or external device. In a demand type pacemaker, a predetermined minimum heart beat rate is established, with its corresponding maximum interval between successive heart beats. The minimum heart beat rate itself may be adjustable or programmable as is generally known in the art, but once it is set the pacemaker will operate in accordance with the selected or programmed rate. So long as the rate of the heart remains above the minimum rate, the stimulating pulse generating circuits are inhibited or reset and the pacemaker does not deliver any stimulating pulses to the heart. However, if the time interval between successive heart beats becomes greater than the selected amount corresponding to the minimum rate, the pacemaker delivers a stimulating pulse to the heart so as to support the heart at or above the preselected minimum rate. In order to provide the demand mode of operation, sensing means are provided for detecting a spontaneous ventricular depolarization. The sensing means generally includes a sense amplifier having an input connected to the ventricular stimulating electrode so that the electrical activity associated with a ventricular depolarization is picked up by the ventricular electrode and conveyed back to the pacemaker where it is applied to a sense amplifier. The sense amplifier generally contains electrical filtering networks which tailor the frequency of response of the amplifier for good sensitivity to the QRS wave complex of the electrocardiogram while rejecting other portions of the electrocardiogram.
Additionally, in the case of an atrial-ventricular pacemaker, the sense amplifier must be capable of rejecting the atrial stimulating pulse delivered by the pacemaker. The atrial stimulation pulse, delivered by the pacemaker through a separate lead to the atrium of the heart, may be conducted through body tissues and picked up by the ventricular lead, from which it is conducted to the sense amplifier. The magnitude of the atrial pulse thus picked up is typically many times that of the QRS wave, complicating the design of the sense amplifier discriminating circuits. Usually, a combination of electronic filtering to adjust the frequency response of the sense amplifier and establishing a minimum lead separation between the locations of the atrial and ventricular stimulation electrodes in the heart is required for proper operation. Unfortunately, this may require tuning the frequency response of the sense amplifier to less than optimum for the QRS wave in order to provide adequate rejection of the atrial pulse. A further disadvantage is that lead separation of electrodes within the heart may be difficult to achieve and maintain, especially if there is a possibility that a lead may be dislodged or moved slightly subsequent to the implantation.